Boron-substituted lithium compounds, active electrode materials, batteries and electrochrome devices

ABSTRACT

Lithium insertion compound having the following formula (I):
 
Li α M β M1 v M2 w M3 x M4 y M5 z B γ (XO 4−ε Z ε ) 1   (I)
         M is selected from V 2+ , Mn 2+ , Fe 2+ , Co 2+  and Ni 2+ ;   M1 is selected from Na +  and K + ;   M2 is selected from Mg 2+ , Zn 2+ , Cu 2+ , Ti 2+ , and Ca 2+ ;   M3 is selected from Al 3+ , Ti 3+ , Cr 3+ , Fe 3+ , Mn 3+ , Ga 3+ , and V 3+ ;   M4 is selected from Ti 4+ , Ge 4+ , Sn 4+ , V 4+ , and Zr 4+ ;   M5 is selected from V 5+ , Nb 5+ , and Ta 5+ ;   X is an element in oxidation state m, exclusively occupying a tetrahedral site and coordinated by oxygen or a halogen, which is selected from B 3+ , Al 3+ , V 5+ , Si 4+ , P 5+ , S 6+ , Ge 4+  and mixtures thereof;   Z is a halogen selected from F, Cl, Br and I;   the coefficients α, β, v, w, x, y, z, γ and ε are all positive and satisfy the following equations:
 
0≦α≦2  (1);
 
1≦β≦2  (2);
 
0&lt;γ  (3);
 
0≦α≦2  (3);
 
0≦ε≦2  (4);
 
α+2β+3γ+v+2 w +3 x +4 y +5 z+m =8−ε  (5); and
       

               0   &lt;     γ     β   +   v   +   w   +   x   +   y   +   z       ≤   0.1     ,         
preferably,
 
     
       
         
           
             0 
             &lt; 
             
               γ 
               
                 β 
                 + 
                 v 
                 + 
                 w 
                 + 
                 x 
                 + 
                 y 
                 + 
                 z 
               
             
             ≤ 
             
               0.05 
               . 
             
           
         
       
     
     Methods for preparing these compounds. 
     Active materials of electrodes, in particular of positive electrodes containing said compounds, and batteries and electrochromic devices using these compounds.

This application is a national phase application of PCT Application No.PCT/FR2003/050148 filed on Dec. 2, 2003, which claims the benefit ofFrench Patent Application No. 02 15343 filed on Dec. 5, 2002, which areboth hereby incorporated by reference.

The present invention relates to lithium insertion compounds, moreprecisely it deals with lithium insertion compounds containing boron,substituted by boron, doped with boron, with a polyanionic skeleton.

The invention further relates to active materials of electrodes, inparticular of positive electrodes containing said compounds, as well asbatteries and electrochromic devices using these compounds.

Lithium batteries are increasingly used as self-contained energysources, in particular in portable equipment, such as computers,telephones, organizers, camcorders, etc., where they are graduallyreplacing nickel-cadmium (NiCd) and nickel-metal hydride (NiHM)batteries. This development has occurred because the performance oflithium batteries in terms of energy intensity (Wh/kg, Wh/l) issubstantially superior to that of the two systems mentioned above.

The active electrode compounds used in these batteries are chieflyLiCoO₂, LiNiO₂ and LiMn₂O₄ for the positive electrode, and carbon, suchas graphite or coke, etc., for the negative electrode. The theoreticaland pratical capacities of these compounds are respectively 275 mAh/gand 140 mAh/g for LiCoO₂ and LiNiO₂, and 148 mAh/g and 120 mAh/g forLiMn₂O₄, for an operating voltage of about 4 volts with respect to metallithium.

It has been proved that manganese oxides, and particularly the spinelstructure family Li_(1+x)Mn_(2−x)O₄ (0≦x≦0.33), are capable ofdemonstrating comparable electrochemical performance to that of cobaltand nickel oxides. It also appears that the greater natural abundance ofmanganese, and the lower toxicity of these oxides compared with cobaltand nickel, are an important advantage for their widespread use inbatteries.

In the particular case of LiMn₂O₄, it has nevertheless been demonstratedthat its combined use with the electrolytes formulated for operation inthe neighbourhood of 4 volts with respect to lithium metal, and whichcontain lithium hexafluorophosphate, results in progressive dissolutionof the manganese oxide and consequently a shorter battery lifetime.

Furthermore, two families of compounds used for is electrochemicalreactions have the advantage of being potentially inexepensive andnon-toxic: these are the olivine isotype family and the Nasicon family;it should be observed that the name Nasicon means “sodium (Na)SuperIonic Conductor” and that this compound has, in particular, theformula Na_(x)M₂X₃O₁₂, where M is a transition metal and X is P, Mo, Si,Ge, S, with 0<x<5 and, preferably, x=3.

These two families consist of equivalent elements and only differ in theratio of the number of polyanions to the number of lithium ions and intheir crystal structure. In fact, the olivine isotype family has anorthorhombic crystal lattice and the Nasicon isotype family has arhombohedral lattice.

Materials of an olivine isotype structure with an orthorhombic crystallattice, such as Li_(1−x)FePO₄, for example LiFePO₄ (triphylite) havethe advantage of being potentially inexpensive and non-toxic. In thecase of LiFePO₄, the insertion/extraction of lithium occurs in atwo-phase process at 3.45V/Li⁺/Li, making this compound stable in almostall organic solvents. Moreover, it proves to be much more stable in thecharged state (“FePO₄”) in the presence of electrolyte than theaforementioned oxides, making its use in batteries extremely safe.

However, the major problem of this family of compounds is their lowelectronic and ionic conductivity at ambient temperature. Thisaccordingly limits the kinetics of lithium, insertion/extraction in thehost structure and the use of these compounds at relatively lowcharge/discharge rates.

Furthermore, the compounds of the Nasicon structure, that is with theformula AxM₂ (XO₄)₃, where A is an alkali metal, such as Na, also offeran advantage as an active positive electrode material, thanks inparticular to their high ionic conductivity of the lithium ions.However, like the compounds of an olivine structure, they are poorelectronic conductors, which limits their use.

Moreover, due to the poor electrochemical kinetics, the compounds of thetwo structural families described above cannot be used as activematerials in an electrochromic device.

Document U.S. Pat. No. 6,085,015 describes lithium insertion materialsof the orthosilicate type containing a tetranion SiO₄ ⁴⁻. This isactually a sub-family of olivine, with a silicate group in which thecore metal, that is the metal that participates electronically in theelectrochemical reaction, is doped with various other metals.

The materials of this document have the following general formula:Li_(x)M_(N−(d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[SiO₄]_(t−(p+s+g+v+a+b))[SO₄]_(s)[PO₄]_(p)[GeO₄]_(g)[VO₄]_(v)[AlO₄]_(a)[BO₄]_(b)in which:

-   -   M is Mn²⁺ or Fe²⁺ and mixtures thereof;    -   D is a metal in oxidation state +2 selected from Mg²⁺, Ni²⁺,        Co²⁺, Zn²⁺, Cu²⁺, Ti²⁺, V²⁺, Ca²⁺;    -   T is a metal in oxidation state +3 selected from Al³⁺, Ti³⁺,        Cr³⁺, Fe³⁺, Mn³⁺, Ga³⁺, Zn²⁺ and V³⁺;    -   Q is a metal in oxidation state +4, selected from Ti⁴⁺, Ge⁴⁺,        Sn⁴⁺, and V⁴⁺;    -   R is a metal in oxidation state +5, selected among V⁵⁺, Nb⁵⁺,        and Ta⁵⁺.

All the M, D, T, Q and R are elements occupying octahedral ortetrahedral sites, s, p, g, v, a and b are stoichiometric coefficientsfor S⁶⁺ (sulphate), P⁵⁺ (phosphate), Ge⁴⁺ (germanate), V⁵⁺ (vanadate),Al³⁺ (aluminate) and B³⁺ (borate) respectively occupying tetrahedralsites.

The stoichiometric coefficients d, t, q, r, p, s, v, a and b arepositive and between 0 and 1.

The materials of this document do not provide a significant improvementover the materials with the two formulas mentioned above, that is of theolivine or Nasicon type. In fact, their electronic and ionicconductivities at ambient temperature are low and their electrochemicalkinetics is limited.

Document EP-A2-1 195 827 relates to a method for preparing an activecathode material with the general formula:LiFe_(1−y)MyPO₄

where M is selected from the group consisting of Mn, Cr, Co, Cu, Ni, V,Mo, Ti, Zn, Al, Ga, Mg, B and Nb with 0.05≦X≦1.2 and 0≦γ≦0.8, in whichthe starting materials necessary for the preparation of the material aremixed, the mixture is ground, the mixture obtained after grinding iscompressed to a preset density, and the compressed mixture is sintered.A carbon bearing material is stirred at any one of the steps of thepreparation method. In the examples, as the compound comprising boron,only the compound with the formula LiFe_(0.25)B_(0.75)PO₄ is prepared.

The theoretical calculation shows that these compounds present very lowtheoretical specific capacities, of about 50 mAh/g.

A still unsatisfied need therefore exists for a lithium insertioncompound that has a high electronic conductivity and high ionicconductivity and hence excellent electrochemical kinetics, better in anycase than those of the compounds of the prior art, particularly betterthan those of the compounds of the olivine or Nasicon type, or thecompounds of document U.S. Pat. No. 6,085,015.

A need consequently exists for a lithium insertion compound which can beused at high charge/discharge rates.

A need also exists for a lithium insertion compound which has a hightheoretical specific capacity greater than that of the compounds of theprior art.

These excellent properties of conductivity and electrochemical kineticsmust go hand in hand, in particular, with low cost, low toxicity, highstability in organic solvents and electrolytes, permitting their useover a long period and with high dependability in devices such asbatteries and electrochromic devices.

The goal of the present invention is to supply a lithium insertioncompound which answers the needs and which meets the requirements statedabove.

It is a further goal of the present invention to supply a lithiuminsertion compound that does not present the drawbacks, defects,limitations and disadvantages of the compounds of the prior art, andwhich solves the problems of the compounds of the prior art.

This goal and others are achieved, according to the invention, by alithium insertion compound having the following formula (I):Li_(α)M_(β)M1_(v)M2_(w)M3_(x)M4_(y)M5_(z)B_(γ)(XO_(4−ε)Z_(ε))₁  (I)

-   -   M is an element in oxidation state +2, selected from V²⁺, Mn²⁺,        Fe²⁺, Co²⁺ and Ni²⁺;    -   M1 is an element in oxidation state +1, selected from Na⁺ and        K⁺;    -   M2 is an element in oxidation state +2, selected from Mg²⁺,        Zn²⁺, Cu²⁺, Ti²⁺, and Ca²⁺;    -   M3 is an element in oxidation state +3, selected from Al³⁺,        Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Ga³⁺, and V³⁺;    -   M4 is an element in oxidation state +4, selected from Ti⁴⁺,        Ge⁴⁺, Sn⁴⁺, V⁴⁺, and Zr⁴⁺;    -   M5 is an element in oxidation state +5, selected from V⁵⁺, Nb⁵⁺,        and Ta⁵⁺;    -   X is an element in oxidation state m, where m is an integer, for        example from 2 to 6, exclusively occupying a tetrahedral site        and coordinated by oxygen or a halogen, which is selected from        B³⁺, Al³⁺, V⁵⁺, Si⁴⁺, P⁵⁺, S⁶⁺, Ge⁴⁺ and mixtures thereof;    -   Z is a halogen selected from F, Cl, Br and I;    -   the coefficients α, β, v, w, x, y, z, γ and ε are all positive        and satisfy the following equations:        0≦α≦2  (1);        1≦β≦2  (2);        0<γ  (3);        0≦α≦2  (3);        0≦ε≦2  (4);        α+2β+3γ+v+2w+3x+4y+5z+m=8−ε  (5);        and

$\begin{matrix}{0 < \frac{\gamma}{\beta + v + w + x + y + z} \leq {0.1.}} & (6)\end{matrix}$

Preferred compounds according to the invention are the compound in whichM is Fe²⁺, X is P, and v, W, x, y, z, and ε are equal to 0, that is thecompound with the formula:Li_(α)Fe_(β)B_(γ)PO₄  (II),and the compound in which M is Mn²⁺, X is P, and v, w, x, y, z, and εare equal to 0 that is the compound with the formula:Li_(α)Mn_(β)B_(γ)PO₄  (III)

Also preferably in the formulas (II) and (III), α is 1, and thecompounds (II) and (III) therefore have the following formulas:LiFe_(β)B_(γ)PO₄  (IV)andLiMn_(β)B_(γ)PO₄  (V)in which γ/β≦0.1.

Further preferred compounds are LiFe_(0.95)B_(0.033)PO₄,Li₃Fe_(1.93)B_(0.07)(PO₄)₃ (this formula clearly fits into the range offormula (I) by dividing all the coefficients by 3, but this writing ispreferred because it shows that the olivine structure is not the same asthe Nasicon structure) and LiMn_(0.95)B_(0.033).

The compounds according to the invention can be described as Nasicon orolivine compounds doped with the non-metal boron (Z=5, M=10.81 g/mole).

In fact, the compounds of the olivine family and of the Nasicon familycan be represented by the general formula LiM(XO₄). After doping withboron, the compound according to the invention is obtained, with theformula, for example (I), in which the boron fundamentally, according tothe invention, occupies a cationic site other than those occupied by theX cation or cations of the polyanionic skeleton XO_(4−ε)Z_(ε). In otherwords, the boron occupies a cationic site, to the exclusion of thecationic sites present in the polyanionic entities or skeletons.

Boron may therefore substitute for, or replace, an Li or a metal atom ofM, M₁, M₂, M₃, M₄, M₅, or occupy a vacant site. This substitution, orreplacement, is achieved very easily, because boron has a very smallionic radius.

The material of the invention presents a very particular structure dueto the very specific position occupied by the boron atoms in thisstructure. This structure fundamentally differentiates this material,the compound of the invention, from the compounds of the prior art,particularly the compounds described in document U.S. Pat. No.6,085,015, where boron is exclusively part of the polyanionic entitiesXO₄.

The addition of boron, as carried out in the compound of the invention,serves significantly to increase the conduction of the charge carriersof the basic materials thus modified, and to obtain much higher levelsof conductivity than those of the compounds of the prior art. Suchconductivity levels permit the use of the compounds according to theinvention in a lithium battery, even at high charge/discharge rates. Ithas thus been demonstrated that the compounds according to theinvention, doped with boron, serve to achieve a gain in deliveredcapacity, that is, in available energy, for example of 40%, with acharge or discharge in 10 hours, and, for example, of 65% with a chargeor a discharge in two hours.

Furthermore, basically, the lithium insertion compound according to theinvention is characterized by the fact that the boron is added in aquantity that is less than or equal to 10 atomic %. This essentialcondition is reflected by the fact that in the formula (I), the equation(6)

$0 < \frac{\gamma}{\beta + v + w + x + y + z} \leq 0.1$is satisfied, and preferably,

$0 < \frac{\gamma}{\beta + v + w + x + y + z} \leq {0.05.}$

In general, the ratio reflects a doping and it is therefore advantageousfor it to be as low as possible for a given efficiency.

In document EP-A2-1 195 827 mentioned above, active cathode materialsare described, but in these compounds with the formulaLi_(x)Fe_(1−y)M_(y)PO₄, where M may be B, the coefficient y is from 0 to0.8, while in the examples of this document, only the specific compoundLiFe_(0.25)B_(0.75)PO₄ (ratio of equation (6)=0.75/0.25=3) is explicitlymentioned.

This document therefore discloses a very broad range for the addition ofboron, ranging from 0 (inclusive) to 0.8, hence the ratios of equation(6) are between 0 and 3.

The range claimed for boron doping according to the invention is suchthat this content serves to obtain the ratio of equation (6) lower thanor equal to 0.1, the range according to the invention is hence verynarrow in comparison with the range disclosed in document EP-A2-1 195827. Furthermore, the range according to the invention is very remotefrom the sole and only value of 0.75 (ratio of equation (6) equal to 3)explicitly disclosed in the specific examples of document EP-A2-1 195827.

It has been demonstrated that the compounds according to the invention,which have a boron doping level lying within this very narrow specificrange, present a very high theoretical specific capacity, which is,surprisingly, much higher than that of the compounds possessing a boroncontent lying within the very broad interval of document EP-A2-1 195827. Thus, for example, it has been demonstrated that the theoreticalspecific capacity of the compound LiFe_(0.25)B_(0.75)PO₄, which is theonly compound given as an example in document EP-A2-1 195 827, was aboutthree times lower than the theoretical specific capacity of the compoundLiFe_(0.25)B_(0.05)PO₄ according to the invention, and in which theboron content lies within the very narrow range according to theinvention.

This obviously shows that a novel technical effect, different from anyeffect obtained in the broad boron content range of the abovementionedEuropean patent application, is achieved in the narrow range accordingto the invention.

This effect concerning a considerable increase in the theoreticalspecific capacity is surprising and unexpected. In fact, nothing tendedto imply that by selecting such a specific narrow interval for the borondoping level, such an increase in the theoretical specific capacitywould be obtained.

Without wishing to be bound by any theory, it can be explained, ex postfacto, that the boron substitutes for the active metal. In fact, thespecific capacity is proportional to the number of ions, for example,Fe²⁺, which are converted, for example, to Fe³⁺ ions, during theextraction of the lithium. Hence it can be considered that any decreasein the concentration, for example, of Fe, will be detrimental and thatthe substitution by boron must remain a doping, that is, must be low,that is, of less than about 10%.

The substantial improvement in the kinetics of the electrochemicalreaction obtained with the compounds of the invention in comparison withthe compounds of the prior art, but also, inter alia, the absence oftoxicity, the high stability, the low cost of the compounds of theinvention, makes them particularly adequate for use in lithiumbatteries, and in other devices, such as electrochromic devices.

The invention further relates to a method for preparing the boron-dopedlithium insertion compound with formula (I).

The method consists in reacting the elements necessary for the formationof a compound with an olivine or Nasicon structure with at least oneboron compound, in order to obtain the lithium insertion compound withformula (I), according to the invention. Said boron compound, alsocalled borated (boron-containing) precursor, being a compound with theformula BXO_(4−ε)Z_(ε) (VI), wherein X, Z, and ε have the meaningsalready given above.

In the method according to the invention and fundamentally, the boroncompound or borated precursor is introduced in a very particular form,that is form B (polyanion). In fact, to prevent the boron from occupyingthe polyanion sites in the end product of formula (I), and for thespecific structure of the compound (I) according to the invention to beobtained, the boron, during the synthesis of the compound of formula(I), must be in a form such that it can no longer bond with the oxygenions of the polyanion. This aim is achieved in the method of theinvention, by avoiding the use of borates or boron oxides as boronprecursors, and by specifically using a boron compound in form B(polyanion).

The boron compound or precursor is preferably selected from thecompounds BPO₄, BVO₄, BAsO₄ and 2B₂O₃-3SiO₂ glass and mixtures thereof.

The method for synthesizing the compound of the invention can be a drymethod or a wet method.

These methods, their respective steps, and the operating conditions ofthese steps, are well known to a person skilled in the art and are notdescribed in detail in the present description; for a detaileddescription of these synthesis methods, reference can be made to thefollowing documents: “A Powder Neutron Diffraction Investigation of theTwo Rhombohedral Nasicon Analogues: γ-Na₃Fe₂ (PO₄)₃ and Li₂Fe₂ (PO₄)₃”by C. MASQUELIER et al., Chem. Matter. 2000, 12, 525-532; and “OptimizedLiFePO₄ for Lithium Battery Cathodes” by A. YAMADA et al., Journal ofthe Electrochemical Society, 148 (3), A224-A229 (2001). However, themethod according to the invention is basically characterized by the useof specific precursors which, alone, serve to obtain the specificstructure of the compounds of formula (I) according to the invention.

In other words, the method according to the invention is distinguishedfrom the known methods by the fact that it uses specific borated(boron-containing) precursors of type B (polyanion).

When the method is a dry method, the starting materials, that isessentially the elements for obtaining the compound of the olivine orNasicon structure, necessary for the formation thereof, and the boroncompound or borated precursor, are in powder form and the methodcomprises a heat treatment step.

When the method is a wet method, the starting materials are added to asolvent and the method comprises a crystallization step.

Whether the method is a dry method or a wet method, it is the use of theabove specific boron compounds and borated precursors in the method ofthe invention, which confers on them their specific structure and theiradvantageous properties.

The invention further relates to active materials for electrodes, inparticular for positive electrodes containing one or more compounds,such as described above.

In such active electrode materials, particularly of positive electrodes,the compounds according to the invention can possibly be combined withone or more other active compounds (that is, other than the compounds ofthe invention), such as conventional compounds, like LiCoO₂, LiNiO₂,manganese oxides, in particular, with a spinel structureLi_(1+x)Mn_(2−x)O₄ (where 0≦x≦0.33), for example LiMn₂O₄, compounds ofthe olivine isotype family, such as Li_(1−x)FePO₄, for example LiFePO₄,compounds with the Nasicon structure, the lithium insertion materials ofthe orthosilicate type described in document U.S. Pat. No. 6,085,015,and the materials described in document EP-A2-1 195 827.

The invention further relates to a positive electrode comprising theactive material, such as described above.

Besides the actual active electrode material, a positive electrodeaccording to the invention generally comprises an electronic conductingmaterial, which is preferably carbon in any form, such as carbon black,acetylene black, graphite or coke.

The positive electrode further comprises a polymer binder.

Said polymer binder is generally selected from fluoropolymers,elastomers and cellulose compounds.

The fluoropolymer can be selected, for example, from the vinylidinefluoride polymers and copolymers and the tetrafluoroethylene polymersand copolymers.

The positive electrode generally comprises between 75 and 95% by weightof active material, between 2 and 15% by weight of conducting material,and between 3 and 10% by weight of polymer binder.

To prepare the positive electrode, the active electrode material, theconducting material and the polymer binder dissolved in a solvent, suchas acetone or N-methyl pyrrolidone, are mixed together. The mixture isapplied, for example, by coating a substrate or a conducting material,for example of aluminium, generally in the form of a sheet, and thesubstrate on which the mixture has been applied is dried by heating,possibly under vacuum.

The invention further relates to a battery, such as a lithium battery,comprising said positive electrode.

Such a battery generally comprises, in addition to said positiveelectrode, a negative electrode, a separator, and an electrolyte. Thenegative electrode can be made of a material generally selected fromlithium metal, lithium alloys, a lithium titanate.

The separator is generally made of a microporous polymer selected, forexample, from polyolefins.

Finally, the electrolyte comprises a solvent and a conducting salt; thesolvent is generally selected from ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, ethyl and methylcarbonate, γ-butyrolactone, sulfolane, dialkyl (C₁₋₄) ethers of ethyleneglycol or polyethylene glycol, for example, diethylene glycol,triethylene glycol, tetraethylene glycol, and mixtures thereof.

A preferred solvent is a mixture of ethylene carbonate and dimethylcarbonate.

The conducting salt is a lithium salt generally selected from lithiumhexafluorophosphate, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and mixtures thereof.

The invention finally relates to an electrochromic device comprising thecompound according to the invention.

In such an electrochromic device, the compound or a material comprisingthe compound according to the invention is often in the form of adeposit on a substrate, for example on glass. The passage of current,that is the insertion/extraction of the lithium, modifies the opticalproperties of the material, for example, its colour, making it possibleto obtain a window of which the colour varies. If not, the operation isidentical to that of the battery.

The invention will be better understood from a reading of thedescription that follows, of embodiments of the invention, provided forillustration and non-limiting, with reference to the drawings appendedhereto, in which:

FIG. 1 is a graph showing the galvanostatic cycling curve of thecompound of example 1 (LiFe_(0.95)B_(0.033)PO₄) under two differentcycling conditions (C/10 (0.05 mA/cm²): curve in dotted line; C/2 (0.26mA/cm²): curve in solid line). The y-axis shows the voltage (involts/Li/Li⁺) and the x-axis shows the specific capacity (in mAh/g);

FIG. 2 is a graph showing the variation in specific capacity (in mAh/g)of the compound of example 1 (LiFe_(0.95)B_(0.033)PO₄) as a function ofthe number of cycles under different cycling conditions (by order ofincreasing numbers of cycles C/10, C/1, C/3, C/2, C/1);

FIG. 3 is a graph showing the galvanostatic cycling curve of the undopedcompound LiFePO₄.

EXAMPLE 1

Preparation of the compound according to the invention with the formulaLiFe_(0.95)B_(0.033)PO₄.

Fe (II) orthophosphate is prepared as reaction intermediate byprecipitation in an aqueous phase from FeSO₄.7H₂O in Na₃PO₄ medium, theprecipitate forms immediately. 10.16 g of the orthophosphate Fe₃(PO₄)₂.5H₂O thus obtained is then intimately mixed with 2.76 g of Li₃PO₄and 0.23 g of BPO₄ in a mill under inert atmosphere (Ar or N₂) for atleast 8 hours.

The resulting mixture is then heat-treated under inert atmosphere for 15minutes at a temperature of 600° C.

A boron-doped olivine is obtained with a B/Fe atomic ratio of 0.035 withthe formula LiFe_(0.95)B_(0.033)PO₄.

EXAMPLE 2

Preparation of the compound according to the invention with the formulaLiMn_(0.95)B_(0.033)PO₄.

15.47 g of manganese phosphate (III) of the hureaulite form (Mn₅(PO₄)₂(PO₃OH)₂.4H₂O) is intimately mixed with 2.52 g of MnCO₃.xH₂Ox=0.2, 4.92 g of Li₃PO₄ and 0.40 g of BPO₄ in a mill under inertatmosphere (Ar or N₂) for at least 8 hours.

The resulting mixture is then heat-treated under inert atmosphere for 15minutes at a temperature of 600° C.

A boron-doped olivine is obtained with a B/Mn atomic ratio of 0.035 withthe formula Mn_(0.95)B_(0.033)PO₄.

EXAMPLE 3

Preparation of the compound according to the invention with the formulaLi₃Fe_(1.93)B_(0.07)(PO₄)₃.

15.0 g of FePO₄.xH₂O (M=188.04 g/mol), 6.74 g of Na₃PO₄ and 0.304 g ofBPO₄ are intimately mixed under air in a planetary mill for 20 hours.This mechanically-chemically activated mixture then undergoes heattreatment at 800° C. in air for 15 minutes. A compound of the Nasiconisotype structure with a formulation approachingNa₃Fe_(1.93)B_(0.07)(PO₄)₃ is obtained. The final compound with theformula Li₃Fe_(1.93)B_(0.07)(PO₄)₃ is then obtained by ion exchange fromNa₃Fe_(1.93)B_(0.07)(PO₄)₃ in a concentrated solution of LiNO₃ in H₂Owith Li_(solution)/Na_(solid)>10 for 1 day.

EXAMPLES 4 EXAMPLE 4A

Fabrication of a lithium battery of which the positive electrodecomprises the compound according to the invention prepared in example 1.

a) Fabrication of the Positive Electrode

-   -   The product obtained in example 1 is mixed with 80% by weight of        acetylene black (Super P, MMM Carbon, Belgium) (10%) and        polyvinylidene fluoride (Solef 6020, Solvay, Belgium) (10%)        dissolved in N-methyl-pyrrolidone.    -   The mixture is then applied to an aluminium sheet, then dried at        60° C., and then heated to 100° C. under vacuum.

b) Fabrication of the Battery

The positive electrode thus prepared is introduced into a “buttonbattery” type cell format 2432. The negative electrode consists of asheet of battery grade lithium (Chemetall-Foote Corporation, USA). Theseparator consists of a film of microporous polypropylene (Celgard 3200,Aventis). The electrolyte used consists of ethylene carbonate, dimethylcarbonate and lithium hexafluorophosphate (LiPF₆) (ElectrolyteSelectipur LP30, Merck, Federal Republic of Germany).

c) Tests Performed with the Battery

As may be observed in FIG. 1, at 25° C., the battery thus fabricatedoperates between 4.5 V and 2.0 V and permits the reversibleextraction/insertion of lithium corresponding to above 140 mAh/g ofpositive active compound under C/2 cycling conditions, that is with acharge or discharge in two hours (curve in solid line); and at about 145mAh/g at C/10, that is, with a charge or a discharge in 10 hours (curvein continuous line).

In other words, the C/2 charge/discharge cycling serves to obtain aspecific capacity of 140 mAh/g.

FIG. 2 shows the variation in specific capacity (mAh/g) of the compoundLiFe_(0.95)B_(0.033)PO₄ proposed in example 1, as a function of thenumber of cycles and under various cycling condition (C/10, C/1, C/3,C/2, C/1).

It may be observed in FIG. 2 that the specific capacity obtained underslow cycling (C/10) is relatively unaffected by the increase in cyclingrate (C/1).

EXAMPLE 4B

A battery is fabricated in the same way as in example 4A, the onlydifference being the use, in the positive electrode, of the compoundaccording to the invention prepared in example 2. The characteristics ofthe battery, particularly as regards the specific capacity, are similarto those of the battery of example 4A.

EXAMPLE 4C

A battery is fabricated in the same way as in example 4A, the onlydifference being the use, in the positive electrode, of the compoundaccording to the invention prepared in example 3. The characteristics ofthe battery, particularly as regards the specific capacity, are similarto those of the battery of example 4A.

EXAMPLE 5 COMPARATIVE

In this example, a lithium battery is fabricated in the same way as inexample 4, in which however the positive electrode consists of LiFePO₄not doped with boron, that is, with a compound not according to theinvention.

The galvanostatic cycling curve of the non-doped compound LiFePO₄ isshown in FIG. 3. The specific capacity of this boron-free material is 80mAh/g given by FIG. 3, for C/7 cycling and corresponding to a currentdensity of 17 μA/cm² imposed on the electrode.

This value of 80 mAh/g, obtained for C/7 cycling, which is much lessconstraining than C/2, should be compared with the specific capacity of140 mAh/g obtained with the compound according to the invention inexample 4A with a constraining C/2 cycling.

EXAMPLE 6 COMPARATIVE

In this example, a lithium battery is fabricated in the same way as inexample 4, in which however the positive electrode consists of LiMnPO₄not doped with boron.

Table I below shows the comparison of the electrochemical performance asregards the delivered capacity (available energy at 25° C.) of thebatteries of examples 4A, 5 and 6.

TABLE I Delivered capacity Compound Cycling (mAh/g) Example 5 LiFePO₄C/10 85 (comparative) Not doped by B C/2 50 Example 6 LiMnPO₄ C/10 80(comparative) Not doped by B Example 4A LiFe_(0.95)B_(0.033)PO₄ C/10 145(invention) C/2 140

It appears from the table that the boron doping procures a 40% increasein capacity at C/10 and a 65% gain at C/2.

The faster the cycling conditions, the greater the increase in capacityobtained, due to the boron doping of the compound according to theinvention.

This table clearly demonstrates that the lithium insertion kinetics isimproved by the boron doping of the compounds according to theinvention.

Equivalent improvements in the properties are obtained with thebatteries of examples 4B (compound according to the invention of example2) and 4C (compound according to the invention of example 3).

1. A lithium insertion compound having the following formula (I):Li_(α)M_(β)M1_(v)M2_(w)M3_(x)M4_(y)M5_(z)B_(γ)(XO_(4−ε)Z_(ε))  (I) M isan element in oxidation state +2, selected from the group consisting ofV²⁺, Mn²⁺, Fe2+, Co²⁺ and Ni²⁺; M1 is an element in oxidation state +1,selected from the group consisting of Na⁺ and K⁺; M2 is an element inoxidation state +2, selected from the group consisting of Mg²⁺, Zn²⁺,Cu²⁺, Ti²⁺, and Ca²⁺; M3 is an element in oxidation state +3, selectedfrom the group consisting of Al³⁺, Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Ga³⁺, andV³⁺; M4 is an element in oxidation state +4, selected from the groupconsisting of Ti⁴⁺, Ge⁴⁺, Sn⁴⁺, V⁴⁺, and Zr⁴⁺; M5 is an element inoxidation state +5, selected from the group consisting of V⁵⁺, Nb⁵⁺, andTa⁵⁺; X is an element in oxidation state m, where m is an integer, from2 to 6, exclusively occupying a tetrahedral site and coordinated byoxygen or a halogen, which is selected from the group consisting of B³⁺,Al³⁺, V⁵⁺, Si⁴⁺, P⁵⁺, S⁶⁺, Ge⁴⁺ and mixtures thereof; Z is a halogenselected from the group consisting of F, Cl, Br and I; the coefficientsα, β, v, w, x, y, z, γ and ε are all positive or equal 0 and satisfy thefollowing equations:0≦α≦2  (1);1≦β≦2  (2);0<γ  (3);0≦ε≦2  (4);α+2β+3γ+v+2w+3x+4y+5z+m=8−ε  (5); and$0 < \frac{\gamma}{\beta + v + w + x + y + z} \leq {0.1.}$
 2. Thecompound according to claim 1, in which M is Fe²⁺, X is P, and v, w, x,y, z, and ε are equal to 0 and which has the following formula (II):Li_(α)Fe_(β)B_(γ)PO₄  (II).
 3. The compound according to claim 1, inwhich M is Mn²⁺, X is P, and v, w, x, y, z, and ε are equal to 0 andwhich has the following formula (III):Li_(α)Mn_(β)B_(γ)PO₄  (III).
 4. The compound according to claim 2, inwhich α=1 and which has the following formula (IV):LiFe_(β)B_(γ)PO₄  (IV) with γ/β≦0.1.
 5. The compound according to claim3, in which α=1 and which has the following formula (V):LiMn_(β)B_(γ)PO₄  (V) with γ/β≦0.1.
 6. The compound according to claim4, which is LiFe_(0.95)B_(0.033)PO₄.
 7. The Compound according to claim4, which is Li₃Fe_(1.93)B_(0.07)(PO₄)₃.
 8. The compound according toclaim 5, which is LiMn_(0.95)B_(0.033)PO₄.
 9. The lithium insertioncompound of claim 1, wherein the coefficients α,β, v,w,x,y,z,γ and ε areall positive or equal 0 and satisfy the following equations:0≦α≦2  (1);1≦β≦2  (2);0<γ  (3);0≦ε≦2  (4);α+2β+3γ+v+2w+3x+4y+5z+m=8−ε  (5); and$0 < \frac{\gamma}{\beta + v + w + x + y + z} \leq {0.05.}$
 10. A methodfor preparing a lithium insertion compound according to claim 1, inwhich the elements necessary for the formation of a compound with anolivine or Nasicon structure are reacted with at least one boroncompound with the formula BXO_(4−ε)Z_(ε) (VI) where X, Z and ε have themeanings already given in claim 1, to yield the lithium insertioncompound having formula (I).
 11. The method according to claim 10, inwhich said boron compound is selected from the group consisting BPO₄,BVO₄, BAsO₄, 2B₂0₃-3SiO₂ glass and mixtures thereof.
 12. The methodaccording to claim 10, which is a wet method, in which the elementsnecessary for the formation of the compound with a Nasicon or olivinestructure and the boron compound are in powder form and which comprisesa heat treatment step.
 13. The method according to claim 10, which is awet method, in which the elements necessary for the formation of thecompound with a Nasicon or olivine structure and the boron compound areadded to a solvent and which comprises a crystallization step.
 14. Anactive electrode material containing one or more compounds according toclaim
 1. 15. A positive electrode comprising the active materialaccording to claim
 14. 16. A battery comprising the electrode accordingto claim
 15. 17. The active electrode material of claim 14 furthercomprising one or more other active compounds, wherein the one or moreother active compounds are selected from the group consisting of LiCoO₂,LiNiO₂, manganese oxides, compounds of the olivine isotype family,compounds having the Nasicon structure, and lithium insertion materialsof orthosilicate type.
 18. The active electrode material of claim 17,wherein the manganese oxides have a spinel structure Li_(1+x)Mn_(2−x)O₄(with 0≦x≦0.33) and the compounds of the olivine isotype family have astructure Li_(1−x)FePO₄.
 19. The active electrode material of claim 18,wherein the manganese oxides comprise LiMn₂O₄ and the compounds of theolivine isotype family comprise LiFePO₄.
 20. An electrochromic devicecomprising the compound according to claim 1.